Color theme edit frame must have an accessible name

An error was reported by automated WCAG testing:

Validation in the test method [validateColorThemeFeatures]
Check for [org.xwiki.flamingo.test.po.EditThemePage] at http://xwikiweb:8080/xwiki/bin/edit/FlamingoThemes/validateColorThemeFeatures?template=FlamingoThemesCode.ThemeTemplate&title=validateColorThemeFeatures&parent=FlamingoThemes.WebHome&xhidden=1&form_token=shFxIIewaj3farn1oMEk4w.
Found [2] items

1: Frames must have an accessible name
Description: Ensures <iframe> and <frame> elements have an accessible name
Help URL: https://dequeuniversity.com/rules/axe/4.6/frame-title?application=axeAPI
Help: Frames must have an accessible name
Impact: serious
Tags: cat.text-alternatives, wcag2a, wcag412, section508, section508.22.i

HTML element:
<iframe src="/xwiki/bin/view/Main/?colortheme=no" id="iframe" class="col-xs-12"></iframe>
Selector: iframe
Fix any of the following:
Element has no title attribute
aria-label attribute does not exist or is empty
aria-labelledby attribute does not exist, references elements that do not exist or references elements that are empty
Element's default semantics were not overridden with role="none" or role="presentation"

See the image below for an in-situation view of the issue:

The frame doesn't have a label, a title or an aria-label. This should be fixed so that assistive technologies can read it out properly.